feat(chat): enable HushChat by default + legacy (non-mnemonic) wallet support

Flip DRAGONX_ENABLE_CHAT to default ON (a fresh configure now builds chat in),
and make full-node chat work for every wallet + any daemon.

Legacy compat: chat identity is derived from the wallet's mnemonic
(z_exportmnemonic) for a portable, SDXLite-compatible identity — but a legacy
random-seed wallet has no mnemonic, and released daemons don't have that RPC at
all. Provisioning now falls back to a stable z-address's spending key
(z_exportkey) in those cases: a functional, wallet-local identity that works on
any daemon. Since a chat identity is local (peers learn your public key from
your message headers, not your derivation), interop is unaffected; only
cross-client portability needs the mnemonic. The spending key is an in-memory
KDF input over a key the wallet already holds, wiped after use — no new exposure.

Stability: the chosen chat z-address (the reply-to in headers AND the legacy
identity source) is now persisted in settings (chat_reply_zaddr), so the
identity + reply address don't shift when new addresses are generated.
chatReplyZaddr() picks the smallest spendable z-addr once and reuses it.

CLAUDE.md updated to reflect the default flip. Verified: Linux + Windows build
with chat ON, fresh-configure default confirmed ON, ctest 100%, hygiene clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/config/settings.cpp

@@ -146,6 +146,7 @@ bool Settings::load(const std::string& path)
         loadScalar(j, "address_explorer_url", address_explorer_url_);
         loadScalar(j, "language", language_);
         loadScalar(j, "skin_id", skin_id_);
+        loadScalar(j, "chat_reply_zaddr", chat_reply_zaddr_);
         loadScalar(j, "acrylic_enabled", acrylic_enabled_);
         loadScalar(j, "acrylic_quality", acrylic_quality_);
         loadScalar(j, "blur_multiplier", blur_multiplier_);
@@ -401,6 +402,7 @@ bool Settings::save(const std::string& path)
     j["address_explorer_url"] = address_explorer_url_;
     j["language"] = language_;
     j["skin_id"] = skin_id_;
+    j["chat_reply_zaddr"] = chat_reply_zaddr_;
     j["acrylic_enabled"] = acrylic_enabled_;
     j["acrylic_quality"] = acrylic_quality_;
     j["blur_multiplier"] = blur_multiplier_;